1. The Field of the Invention
The present invention is directed to processes for reducing nitrogen oxide emissions in combustion systems. More specifically, the present invention provides methods of decreasing the concentration of nitrogen oxides in flue gases emitted to the atmosphere from stationary combustion systems such as boilers, furnaces and incinerators.
2. The Relevant Technology
Nitrogen oxides are the major air pollutants emitted by boilers, furnaces, engines, incinerators, and other combustion sources. Nitrogen oxides include nitric oxide (NO), nitrogen dioxide (NO.sub.2), and nitrous oxide (N.sub.2 O). Total NO+NO.sub.2 concentration is usually referred to as NO.sub.x. Combustion sources produce nitrogen oxides mainly in the form of NO. Some NO.sub.2 and N.sub.2 O are also formed, but their concentrations are typically less than 5% of the NO concentration, which is generally in the range of about 200-1000 ppm. Nitrogen oxides are the subject of growing concern because they are toxic compounds, and are precursors to acid rain and photochemical smog. Nitrous oxide also contributes to the greenhouse effect.
Combustion modifications such as low NO.sub.x burners (LNB) and overfire air (OFA) injection provide only modest NO.sub.x control, reducing NO.sub.x concentrations by about 30-50%. However, their capital costs are low and, since no reagents are required, their operating costs are near zero. For deeper NO.sub.x control, Selective Catalytic Reduction (SCR), reburning, Advanced Reburning (AR), or Selective Non-Catalytic Reduction (SNCR) can be used in conjunction with low NO.sub.x burners and overfire air injection, or they can be installed as stand-alone systems.
Currently, SCR is the commercial technology with the highest NO.sub.x control efficiency. With SCR, NO.sub.x is reduced by reactions with nitrogenous reducing agents (N-agents) such as ammonia, urea, etc., on the surface of a catalyst. The SCR systems are typically positioned at a temperature of about 700.degree. F. in the exhaust stream. Although SCR can relatively easily achieve 80% NO.sub.x reduction, it is far from an ideal solution for NO.sub.x control. The size of the catalyst bed required to achieve effective NOx reduction is quite large, and use of this large catalyst, with its related installation and system modification requirements, is expensive to implement. In addition, catalyst deactivation, due to a number of mechanisms, typically limits catalyst life to about four years for coal-fired applications. The spent catalysts are toxic and pose disposal problems.
The reduction of NO.sub.x can proceed without a catalyst at a higher temperature. This is the SNCR process. It is effective over a narrow range of temperatures, or "temperature window" centered at about 1800.degree. F. where the N-agent forms NH.sub.i radicals which react with NO. Under ideal laboratory conditions, deep NO.sub.x control can be achieved; however, in practical, full-scale installations, the non-uniformity of the temperature profile, difficulties of mixing the N-agent across the full combustor cross section, limited residence time for reactions, and ammonia slip (unreacted N-agent) limit SNCR's effectiveness. Typically, NO.sub.x control via SNCR is limited to 40-50%. Thus, while SNCR does not require a catalyst and hence has a low capital cost compared to SCR, it does not provide high efficiency NO.sub.x control. The most common SNCR N-agents are ammonia and urea, and the corresponding methods are called "Thermal DeNO.sub.x " and "NO.sub.x OUT."
The Thermal DeNO.sub.x process is described in detail in U.S. Pat. No. 3,900,554 to Lyon, and in Lyon and Hardy, "Discovery and Development of the Thermal DeNO.sub.x Process," Ind. Eng. Chem. Fundam., 25, 19 (1986). When ammonia is injected into combustion flue gas containing NO and oxygen at temperatures between about 1500 and 2000.degree. F., a series of chemical reactions occurs and NO is converted to molecular nitrogen. The reaction is believed to start with formation of NH.sub.2 radicals by reaction of ammonia with OH, O or H atoms: EQU NH.sub.3 +OH.fwdarw.NH.sub.2 +H.sub.2 O EQU NH.sub.3 +O.fwdarw.NH.sub.2 +OH EQU NH.sub.3 +H.fwdarw.NH.sub.2 +H.sub.2
The main elementary reaction of the NO to N.sub.2 conversion is then: EQU NH.sub.2 +NO.fwdarw.N.sub.2 +H.sub.2 O
Another SNCR additive is urea, (NH.sub.2).sub.2 CO, which is disclosed in U.S. Pat. No. 4,208,386 to Arand et al., and is used in the NO.sub.x OUT process. When added to combustion flue gases, urea is rapidly thermally decomposed to NH.sub.3 and HNCO: EQU (NH.sub.2).sub.2 CO.fwdarw.NH.sub.3 +HNCO
Thus, the mechanism of urea reduction of NO.sub.x includes the reactions of NH.sub.3 described above, as well as reaction of HNCO. The most important HNCO reactions with radicals are: EQU HNCO+H.fwdarw.NH.sub.2 +CO and EQU HNCO+OH.fwdarw.NCO+H.sub.2 O
As in the Thermal DeNO.sub.x process, NH.sub.2 radicals can either remove NO: EQU NH.sub.2 +NO.fwdarw.N.sub.2 +H.sub.2 O
or form NO by reaction with HNO radicals. NCO radicals can remove NO to form N.sub.2 O: EQU NCO+NO.fwdarw.N.sub.2 O+CO
and then CO and N.sub.2 O molecules are oxidized by OH and H, respectively: EQU CO+OH.fwdarw.CO.sub.2 +H EQU N.sub.2 O+H.fwdarw.N.sub.2 +OH
Thus, the process has a similar narrow temperature window as NH.sub.3 injection, but can be complicated by N.sub.2 O formation. The SNCR temperature window could be broadened to lower temperatures if an alternative source of OH radicals could be found. Attempts to do this have included addition of hydrogen or hydrogen peroxide to ammonia, alcohols to urea, etc. The action of most additives is to shift the temperature at which the de-NO.sub.x reactions are optimum, rather than to broaden the de-NO.sub.x temperature window. However, U.S. Pat. No. 5,270,025 to Ho et al. discloses several salt additives that considerably broaden the temperature window of the Thermal DeNO.sub.x process.
An alternative to controlling NO.sub.x emissions by SCR or SNCR processes is reburning. Reburning is a method of controlling NO. emissions via fuel staging. The main portion of the fuel (80-90%) is fired through conventional burners with a normal amount of air (about 10% excess) in a main combustion zone. The combustion process forms a definite amount of NO.sub.x . Then, in a second stage, the rest of the fuel (the reburning fuel) is added at temperatures of about 2000-2600 .degree. F. into the secondary combustion zone, called the reburning zone, to maintain a fuel-rich environment. In this reducing atmosphere both NO.sub.x formation and NO.sub.x removal reactions occur. Experimental results indicate that in a specific range of conditions (equivalence ratio in the reburning zone, temperature and residence time in the reburning zone), the NO.sub.x concentrations can typically be reduced by about 50-70%. In a third stage, air is injected (overfire air, or OFA) to complete combustion of the fuel. Addition of the reburning fuel leads to the rapid oxidation of a portion of the fuel by oxygen to form CO and hydrogen.
The reburning fuel provides a fuel-rich mixture with certain concentrations of carbon containing radicals: CH.sub.3, CH.sub.2, CH, C, HCCO, etc. These active species can participate either in the formation of NO precursors in reactions with molecular nitrogen or in reactions with NO. Many elementary steps can share responsibility for NO reduction, and there is no commonly accepted opinion about their relative importance. The carbon containing radicals (CH.sub.i) formed in the reburning zone are capable of reducing NO concentrations by converting NO to various intermediate species with C--N bonds. These species are reduced in reactions with different radicals into NH; species (NH.sub.2, NH, and N), which react with NO to form molecular nitrogen. Thus, NO can be removed by reactions with two types of species: CH.sub.i and NH.sub.i radicals. The OFA added in the last stage of the process oxidizes remaining CO, H.sub.2, HCN, and NH.sub.3 and unreacted fuel and fuel fragments. The reburning fuel can be coal, gas or other fuels.
The Advanced Reburning (AR) process is a synergistic integration of reburning and SNCR, and is disclosed in U.S. Pat. No. 5,139,755 to Seeker et al. In the AR process, an N-agent is injected along with the OFA, and the reburning system is adjusted to optimize NO.sub.x reduction by the N-agent. By adjusting the reburning fuel injection rate to achieve near stoichiometric conditions (instead of the fuel rich conditions normally used for reburning), the CO level is controlled and the temperature window for effective SNCR chemistry is considerably broadened. With AR, the NO.sub.x reduction achieved from the N-agent injection is increased. Furthermore, the widening of the temperature window provides flexibility in locating the injection system, and NO.sub.x control should be achievable over a broad boiler operating range.
The Advanced Reburning process provides an approach for increasing the OH concentration to form NH.sub.2 radicals from N-agents. It incorporates the chain branching reaction of CO oxidation into the process. When CO reacts in the presence of oxygen and water vapor (H.sub.2 O), it creates free radicals including H, OH, O and HO.sub.2. Thus, if a controlled amount of CO from the reburning zone can be introduced at the point of N-Agent injection, the low temperature limitation of the window can be broadened and the NO.sub.x reduction enhanced.
Experimental studies have demonstrated two approaches for addition of OFA in reburning to prepare specific SNCR conditions. (Chen et al., "Advanced Non-Catalytic Post Combustion NO.sub.x Control," Environ. Progress, 10, 182 (1991)). One approach is to split the OFA addition and inject it in two stages so that the bulk of the oxidation is completed at the conventional OFA injection stage while a moderate amount of CO is left for burnout at a second injection stage at lower temperature where conditions are more favorable for DeNO.sub.x reactions. In an alternative approach, the reburning zone is deliberately de-tuned by increasing the stoichiometry to about 0.98-1.0. This allows a significant reduction in the reburning fuel flow, and eliminates one of the air injection stages. The basic AR process, i.e., CO-promoted N-Agent injection, shows that the temperature window can be broadened and NO removal efficiency increased if both CO and O.sub.2 concentrations are controlled to fairly low values (CO on the order of about 1000 ppm, and O.sub.2 at less than about 0.5 percent). At the point of air addition, CO and O.sub.2 are both low because of the close approach to SR=1.0.
U.S. Pat. No. 5,756,059 to Zamansky et al. discloses an improved Advanced Reburning process in which the N-agent can be injected under fuel rich conditions or at two injection locations, one each under fuel-rich and fuel-lean conditions, for deeper NO.sub.x control. The N-agent can be injected with or without promoters at one or two locations in the reburning zone, along with OFA or downstream in the burnout (SNCR) zone. The promoters are water-soluble inorganic salts that can be added to aqueous N-agents, or to solid, liquid or gaseous N-agents, and injected along with the N-agents to enhance the N-agent efficiency. In pilot scale AR experiments, NO.sub.x reduction of up to 95% was achieved. The estimated total cost of NO.sub.x control for AR systems is approximately half of that for SCR.
The chemistry of AR is no different than that for basic reburning and SNCR, and the reactions discussed above proceed. The critical difference is how the two sets of chemical reactions are synergistically integrated together. The final OFA initiates the oxidation of CO from the reburning zone: EQU CO+OH.fwdarw.CO.sub.2 +H EQU H+O.sub.2.fwdarw.OH+O EQU O+H.sub.2 O.fwdarw.OH +OH
This chain branching sequence provides additional OH radicals to initiate the NH.sub.3 oxidation sequence: EQU NH.sub.3 +OH.fwdarw.NH.sub.2 +H.sub.2 O
NH.sub.2 +NO.fwdarw.N.sub.2 +H.sub.2 O
While prior systems are capable of controlling NO.sub.x emissions, even the most effective systems are still complex. In addition, effective NO.sub.x reduction systems can be expensive to implement, operate and maintain. Thus, there is a need for simpler, less expensive, and effective processes for reducing the NO.sub.x concentration in combustion flue gases.